High frequency oscillator, high frequency welding system and method for controlling the frequency using said type of high frequency oscillator

ABSTRACT

A high-frequency oscillator includes an electric resonant circuit, and a high-frequency welding system and a method for controlling the frequency uses a high-frequency oscillator, in particular in a high-frequency welding system. The electric resonant circuit includes at least one electronic component having an inductance and at least one capacitor having a capacitance. At least one additional magnetic coil is associated with the electronic component and can electronically influence the inductance of the electronic component.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional of and Applicant claims priority under35 U.S.C. §§ 120 and 121 of U.S. application Ser. No. 14/787,309 filedon Oct. 27, 2015, which application is a national stage applicationunder 35 U.S.C. § 371 of PCT Application No. PCT/DE2014/000176 filed onApr. 8, 2014, which claims priority under 35 U.S.C. § 119 from GermanPatent Application No. 10 2013 008 730.9 filed on May 24, 2013 andGerman Patent Application No. 10 2013 021 062.3 filed on Dec. 18, 2013,the disclosures of each of which are hereby incorporated by reference. Acertified copy of priority German Patent Application No. 10 2013 008730.9 and a certified copy of priority German Patent Application No. 102013 021 062.3 are contained in parent U.S. application Ser. No.14/787,309. The International Application under PCT article 21(2) wasnot published in English.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The invention relates to a high-frequency oscillator. The inventionfurther relates to a high-frequency welding system for high frequencywelding of plastic films with at least one high frequency oscillator ofsaid type and to a method for controlling the frequency using said typeof high frequency oscillator.

2. Description of the Related Art

For high frequency welding of plastics, more specifically plastic films,high frequency welding systems are customarily used. In the process, theplastics are passed between electrodes and heated up by means of a highfrequency electromagnetic alternating field. The plastic films are thuswelded together at their points of contact. During the process, a workfrequency is set by means of high frequency oscillators that is asstable as possible and lies for example in the range of 25 to 30megahertz.

The high frequency oscillators used to that end are built as so-calledsolid-state generators and therefore do not have vacuum tubes forfrequency conversion but semi-conductor components for converting thedirect power into a high frequency alternating power. The high frequencyoscillators operate virtually wear-free and are relatively insensitiveto mechanical stresses.

However, these high frequency oscillators have relatively low frequencystability, i.e. a low quality factor. More specifically in case of achange of load, the output frequency may thus be detuned. As aconsequence, the oscillator frequency changes more than is authorized bylegislative authorities. Accordingly, it is necessary to control thefrequency of the high frequency oscillator.

Usually, this control is achieved by mechanically influencing theinductance of the corresponding electronic component of the electricresonant circuit, which, as a rule, is at least formed by a coil. In theprocess, a ferromagnetic core is for example moved more or less farinside the coil, or the generated magnetic field is influenced by way ofa highly electrically conductive core. The inductance can thus becontrolled by way of a corresponding mechanical system driven by anelectric motor and the resonant circuit can be set to a target frequencyeven in case of changes in the load.

However, in case of rapid load changes, the frequency correction cannothappen at sufficiently short notice. In particular, load changes thatare shorter than 300 milliseconds cannot be corrected, the control beingadditionally complicated by a relatively strong tendency to overshoot.Furthermore, the necessary mechanical system is subject to wear.

SUMMARY OF THE INVENTION

The problem underlying the invention is thus to eliminate thedisadvantages of the prior art and in particular to provide apossibility allowing for wear-free frequency control having a highcontrol accuracy even in case of rapid load changes.

In a high frequency oscillator with an electric resonant circuit havingat least one electronic component having an inductance and at least onecapacitor, it is provided according to the invention that at least onemagnetic coil is associated with the electronic component and canelectronically influence the inductance of the electronic component.

The inductance is thus not influenced by mechanically moving aferromagnetic core or by displacing the magnetic field but byelectronically influencing it. In the process, a magnetic field isgenerated with at least one magnetic coil, so that the inductance of theelectronic component is influenced. This allows for a very high-speedfrequency correction even in case of rapid load changes, the inductancebeing influenced without mechanical displacement and thus withoutmechanical wear. Rather, the resonant circuit is tuned magnetically.This results in a very precise frequency control without noticeableovershoot tendency. As a whole, the result is high control accuracy.

It is preferable that the magnetic coil be integrated in a controlcircuit for controlling the frequency of the high frequency oscillator.The magnetic coil is then more specifically operated with a directcurrent. The inductance can be continuously adjusted through a suitablepower supply. The maximum inductance of the electronic component is usedwhen the power to the magnetic coil is cut off. In contrast, in order tooperate the high frequency oscillator at the highest possible frequency,the magnetic coil is supplied with maximum power and the inductance isthus minimized. Thus, by appropriately actuating the magnetic coil, thefrequency of the electric resonant circuit of the high frequencyoscillator can be sensitively controlled.

The magnetic coil is particularly preferably wound from a ring offerromagnetic material. The maximum current that can be fed through themagnetic coil then depends on the moment at which the ring offerromagnetic material reaches saturation.

In a preferred development, the electronic component has at least onecopper tube that is passed through the magnetic coil. In particular,several copper tubes are provided that are passed through one or severalmagnetic coils. This allows controlling higher power outputs. Designingthe electronic component with one or several copper tubes results in ahigh frequency coil of corresponding quality. This also allowsconverting higher power outputs.

In a preferred embodiment, the high frequency oscillator is designed asa high frequency generator. Thereby it is particularly suited forproviding a stable high frequency alternating voltage such as requiredfor example by high frequency welding systems.

The afore-mentioned problem is solved, in accordance with the invention,by a high frequency welding system for high frequency welding ofplastics with at least one high frequency oscillator according to one ofthe claims 1 to 6, in that the high frequency oscillator is integratedin a high frequency circuit and/or a load circuit of the high frequencywelding system. A high frequency alternating voltage is transferred bythe high frequency circuit to the load circuit, and transmitted fromthere to the electrode and counter electrode. The high frequencyoscillator according to the invention then allows a very precise controlof the output frequency of the high frequency circuit, on the one hand,but also of the resonance frequency or impedance in the load circuit.This results in good frequency stabilization in the high frequencycircuit, the high frequency oscillator operating as a high frequencygenerator. A good resonant circuit correction and/or power outputcontrol as well as a precise impedance control can take place in theload circuit.

The high frequency welding system preferably has a higher-levelcontroller for tuning the frequency in the high frequency circuit andfor tuning the resonance frequency in the load circuit. Thereby, highcontrol accuracy is achievable. That way, an optimal operating frequencyof the load circuit can be set, which, where applicable, may even becarried out by way of an automatic search run, during which the systemis operated at various output frequencies by correspondingly graduallychanging the inductance of the high frequency oscillator disposed in thehigh frequency circuit until an optimum is reached.

The afore-mentioned problem is also solved in accordance with theinvention by a method for controlling the frequency using a highfrequency oscillator, more specifically in a high frequency weldingsystem according to one aspect of the invention, in that a frequency ofthe high frequency oscillator is controlled wear-free by electronicallyadapting an inductance. Since no moving parts are required forinfluencing the inductance, no wear can occur. In addition there is noneed to fear an overshoot caused by the inertia of mechanicalcomponents. Thus a very short reaction time of less than 5 millisecondsis achievable and therefore a very fast frequency control.

The inductance is preferably adjusted by applying a direct current tomagnetic coils. This makes it possible to very quickly and verysensitively influence the inductance and thus the output frequency ofthe high frequency oscillator.

An impedance of the electric resonant circuit is preferably adjusted byadapting the inductance. As a result, the range of applications of thehigh frequency oscillator is expanded; it can be used for example in theload circuit.

In a preferred development, the inductance is gradually or continuouslychanged in an automatic search run until an optimum frequency isobtained in the load circuit. Thus, after completion of the automaticsearch run, the high frequency welding system can be operated with anoptimal frequency in the load circuit. Very good welding results arethus reliably and reproducibly achievable.

In an example not shown, the electronic adaptation of the inductance isused for tuning the load circuit of amplifiers. If, for example, anamplifier delivers an output impedance of 50 Ohm, it is accompanied by areal resistance in the load circuit of 50 Ohm. In the prior art, tuningthe amplifier to the load circuit is done with adjustable vacuumvariable capacitors that are adjusted by means of stepper motors.Instead of capacitive tuning, the tuning can also be carried outinductively in accordance with the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following, the invention will be described in more detailed basedon an exemplary embodiment with reference to the drawings. In thedrawings:

FIG. 1 shows a schematic diagram of the high frequency oscillator and

FIG. 2 shows a schematic diagram of a high frequency welding system.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1 shows a schematic diagram of a high frequency oscillator 1comprising an electric resonant circuit 2. In the electric resonantcircuit 2, the electronic component 3 that provides an inductance L is acoil. In addition, the resonant circuit comprises a capacitor 4 having acapacitance C. In its structural principle, the resonant circuit is aconventional LC resonant circuit. In this example, the electroniccomponent 3 is designed as a coil with at least one copper tube.However, it is also possible to provide several coils as well as severalcapacitors.

The electronic component 3 is passed through a ring 5 made of aferromagnetic material, around which a magnetic coil 6 is wound. Themagnetic coil 6 is integrated in a control circuit 8 that comprises acontrollable direct current source 7.

Through appropriate actuation of the direct current source 7, themagnetic coil 6 generates a magnetic field, which is superimposed on thefield of the electronic component 3 and thus influences the inductanceof the electronic component 3. When no current flows through the controlcircuit 8, there is no interference by the magnetic coil 6, so that theinductance of the electronic component 3 is at its maximum. Thefrequency of the electric resonant circuit 2 is thus minimal. When thecontrol circuit 8 is operated in such a manner that the ring made of aferromagnetic material is brought to saturation, a correspondinglystrong magnetic field is generated by the magnetic coil 6 and theinductance of the electronic component 3 is reduced as far as possible.A frequency of the electric resonant circuit 2 is thus very high.Between these two extremes, the inductance of the electronic component 3and thus the resonance frequency of the resonant circuit 2 can beadjusted sensitively and very quickly through appropriate actuation ofthe direct current source 7.

FIG. 2 shows a schematic representation of a high frequency weldingsystem 9 for high frequency welding of plastic films, wherein a highfrequency output oscillation is provided in a high frequency circuit 21by means of a first high frequency oscillator according to the inventionin a high frequency generator 20 and transmitted via a coaxialconnection 10 to a load circuit 11.

In the load circuit 11, there is provided a magnetic resonant circuit12, via which an electrode 13 and a counter electrode 14 are suppliedwith an appropriate high frequency voltage. The material to be welded ispassed between the electrode 13 and the counter electrode 14. In theprocess, it is necessary to tune the system to the pending load, whichis influenced for example by the thickness of the material to be welded.To this end, a second magnetically tuneable resonant circuit accordingto the invention is provided in 12, which comprises a means forelectronically influencing its inductance such as the high frequencyoscillator.

A higher-level controller 15 is provided for tuning the frequency in thehigh frequency circuit 21 by means of the first high frequencyoscillator according to the invention and for adjusting a resonancefrequency in the load circuit 11 by means of the second magneticresonant circuit. Control signals of the higher-level controller aretransmitted via suitable driver elements 16, 17 and amplifiers 18, 19 tothe control circuit 8 for frequency control of the first high frequencyoscillator in the high frequency circuit 21, respectively to acorresponding control circuit 22 of the second magnetic resonant circuitdisposed in the load circuit 11. Thereby a complete control of the highfrequency welding system is possible, thus allowing an optimaladjustment of the operating point.

The design according to the invention allows for a good frequencycorrection in the high frequency oscillator even in case of rapid loadchanges. It is not subject to mechanical wear and has no tendency toovershoot, since it does not have any mechanically movable components.Rather, the frequency of the resonant circuit of the oscillator ischanged purely electronically by appropriately influencing itsinductance. This is done by applying a direct voltage to at least oneadditional magnetic coil, which correspondingly influences theinductance of the electronic component provided in the resonant circuit.

Thus the inductance and therefore the frequency in the resonant circuitchange depending on the actuation of the additional magnetic coil. Thehigh frequency oscillator according to the invention can be usedsimilarly for impedance control, respectively control of the resonancefrequency in the load circuit of a high frequency welding system. Veryhigh control accuracy and high control speed are also achievable there.A potentially higher power consumption can lead to a temperatureincrease, which can be contained by additional cooling for example bymeans of air or water. As a whole, this results in very high frequencystability and a good tuning of the load circuit, an automatic search runfor the optimal operation frequency of the load circuit being alsopossible. The correction of the resonant circuit frequency is carriedout without wear and electronically. An adjustment of the impedance canalso be carried out electronically. In the process, the respectivefrequency of the oscillator is electronically adjusted by magneticcoils.

The invention is not limited to the shown exemplary embodiments and canbe modified in numerous ways. For example, it is possible to use severalmagnetic coils with respectively one ring of ferromagnetic material. Itis also possible to use the high frequency oscillator according to theinvention not only as a high frequency generator for generating highfrequency voltages or currents, but also for example as sensitive highpass or low pass filters.

What is claimed is:
 1. A method for controlling a frequency, the methodcomprising: providing a high frequency oscillator having an electricresonant circuit comprising at least one electronic component having aninductance and at least one capacitor, wherein at least one magneticcoil is associated with the electronic component and with which theinductance of the electronic component is electronically influenceable;and controlling the frequency of the high frequency oscillator wear freeby electronically adjusting the inductance.
 2. The method according toclaim 1, wherein the inductance is adapted by applying a direct currentto at least one magnetic coil (5), which is associated with theelectronic component providing the inductance in the electric resonantcircuit.
 3. The method according to claim 1, wherein an impedance of theelectric resonant circuit is adjusted by adapting the inductance.
 4. Themethod according to claim 1, wherein the inductance is gradually orcontinuously varied in an automatic search run until an optimalfrequency is achieved in a load circuit of a high frequency weldingsystem.
 5. The method according to claim 1, wherein in high frequencywelding systems with high frequency oscillators and/or in high frequencywelding systems with solid state amplifiers, the tuning of the magneticresonant circuit is used as resonance or impedance tuning in the loadcircuit.